Technical Field
The present invention relates generally to nuclear reactors and nuclear fuel assemblies used in the core of nuclear reactors. More specifically, the present invention relates to Canadian Deuterium-Uranium (CANDU) heavy-water reactors, and fuel assemblies for use in the same.
Related Art
FIGS. 1A and 1B depict simplified cross-sectional views of examples of conventional fuel assemblies 10. FIG. 1A depicts a fuel assembly 10 of the PWR type, and FIG. 1B depicts a fuel assembly 10 of the water-cooled water-moderated power rector (VVER) type. In FIG. 1A, the fuel rod assembly 10 comprises fuel rods assembled into a square grid. The PWR fuel assembly 10 of FIG. 1A has fuel rod bundle self-spacing that can be described as having a square cross-sectional shape. In FIG. 1B, the fuel assembly 10 comprises fuel rods arranged into a triangular grid. The VVER fuel assembly 10 of FIG. 1B has fuel rod bundle self-spacing that can be described as having a regular hexagonal cross-section shape.
When these assemblies are fitted into a tube 12, empty segments not used by the fuel rod assembly are formed, as shown by the shaded area 14 located between the tube 12 and the square 14 in FIG. 1A, and between the tube 12 and the hexagon 16 in FIG. 1B. According to embodiments, an assembly in a square grid occupies approximately 63.7% of the area of the circumscribed circle (e.g., tube 12), while an assembly in a triangular grid occupies approximately 82.7% of the area of the circumscribed circle (e.g., tube 12).
It is known to use the empty space to address concerns of fuel rod and assembly swelling during burnup. It is also known to fill these areas with a burnable absorber, etc.